It’s impossible because s1-mod does not support loading custom fastfiles to the extent other clients allow.
To add Chinese you would need to create a font for the language and then translate over possibly 10,000 lines of English localized strings.
It could be automated with AI but knowing how bad English to Chinese can be when AI does it, it sounds like a difficult task. Nonetheless, because you can’t even add Chinese font to the game, you will most likely not be able to do it.
Our clients except IW4x (T7x when it was supported) only support English. English is the language all staff members can speak, so it does not make sense for us to spend resources to expand language support especially when we can’t guarantee basic quality.